A recently developed type of secondary or rechargeable electrical conversion device comprises: (A) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; (B) a cathodic reaction zone containing (i) a cathodic reactant comprising sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversibly reactive with said anodic reactant, and (ii) a porous conductive electrode which is at least partially immersed in said cathodic reactant; and (C) a solid electrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with said anodic and cathodic reaction zones. As used herein the term "reactant" is intended to means both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surface of the electrode by reaction of the cathodic reactant with electrons conducted through the electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the conductive material in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte into the liquid electrolyte and combine to form alkali metal polysulfide near the solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus, electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of ionic and electronic phases, this charging process occurs preferentially in the vicinity of the electrolyte consuming the alkali metal polysulfide and forming molten elemental sulfur.
A number of U.S. patents and U.S. applications, all assigned to Ford Motor Company, the assignee of the invention described and claimed herein, are directed toward the achievement of improved rechargability and increased ampere hour capacity of secondary batteries or cells of the type to which the subject invention is directed. Thus, U.S. Pat. Nos. 3,811,943, 3,951,689 and 3,980,496 are directed to energy conversion devices including secondary batteries or cells which allow or promote improved mass transportation of reactants and reaction products to and from the vicinity of the solid electrolyte and the electrodes during both discharge and charge, thereby increasing the ampere-hour capacity of the secondary battery or cell. U.S. Pat. No. 3,966,492 is directed toward the use of a particular electrode material to improve ampere-hour capacity while U.S. Pat. No. 3,976,503 is directed toward a particular process involving temperature gradients to improve rechargeability of the battery or cell.
U.S. Pat. Nos. 3,985,575 and 3,993,503 claim a combination of particular electrode materials plus improved electrode design to achieve improved charging and discharging characteristics. U.S. Pat. Nos. 4,002,806 and 4,002,807 teach improving the charge/discharge capacity of the battery or cell by including controlled amounts of certain additives in the cathodic reactant.
The invention described and claimed herein provides a simplified alternative to the processes, cell design modifications and material modifications disclosed and claimed in the above identified U.S. patents and patent applications. Unlike the inventions disclosed and claimed in those patents and applications, the invention disclosed and claimed herein does not rely on a modification of the cathodic reaction zone or its means of operation, but rather involves modification of the surface of the cation-permeable barrier. As such, the improvement of the invention may be employed alone to increase rechargeability and resultant ampere-hour capacity or it may be used in conjunction with one or more methods or other improvements for improving rechargeability, including those disclosed and claimed in the patents and applications discussed above.